The invention relates to ramp function generators, in general, and more specifically to a digital ramp function generator for accelerating and/or decelerating a motor drive.
Ramp function generators are important to accelerate, or decelerate, a motor drive from some initial speed to a terminal speed at a controlled rate. This is particularly useful in a rolling mill, in a paper mill, where a strip of material is being unwound from a pay-off reel and rewound on a delivery reel. The tension of the strip is to be maintained constant as a function of varying diameters on the reel, and the motor drives are automatically controlled in speed for that purpose. Accordingly, the rate of change of the speed, depending upon the size of the reels and the nature of the strip, has to be defined and set under varying circumstances by the operator. Moreover, in a rolling mill, where the strip is passed through processing roll stands, the stands are controlled in speed under a master reference with individual stand speed regulators maintaining constant mass flow between the pay-off reel and the delivery reel. In such a case, it is desirable to be able to adjust the speed rate individually at each stand as the overall process requires.
Acceleration of a motor drive was effected at an early stage of the prior art by a motor-operated rheostat. Later, magnetic amplifiers were used with the addition of large capacitors in a feedback loop to generate an integrating function. At the present time, a common approach is to use operational amplifiers to generate an integrating function. In such case, miniature motor-operated rheostats have been associated with the output, which are mounted on printed circuit cards. On the one hand, the motor-operated rheostat offers the advantage over the operational amplifier that, in the hold mode, it does not drift within the limits defined by the reference voltage applied to the rheostat elements. On the other hand, the operational amplifier allows more easily to change the rate of speed and provide rounding in the change of speed at the start, or before a stop. However, operational amplifiers are subject to drift.